feat: stats activity-over-time granularity selector (timeline / day / week / month / year)

Backend adds cyclical distributions ByHour (hour-of-day), ByDOW (weekday), ByDOM
(day-of-month) and ByMonthYr (calendar month), all UTC. The Activity-over-time
card gains a selector: Timeline keeps the chronological month trend; Day/Week/
Month/Year show the hour/weekday/day/month distribution as bars.

frontend/src/components/StatsPanel.tsx

@@ -32,6 +32,7 @@ type Stats = {
   confirmed_lotw: number; confirmed_eqsl: number; confirmed_qsl: number; confirmed_any: number;
   by_mode: Bucket[]; by_band: Bucket[]; by_band_category: BandCat[]; by_operator: Bucket[]; by_station: Bucket[];
   by_continent: Bucket[]; top_entities: Bucket[]; by_year: Bucket[]; by_month: Bucket[];
+  by_hour: Bucket[]; by_dow: Bucket[]; by_dom: Bucket[]; by_month_yr: Bucket[];
   // Period / contest metrics.
   window_start: string; window_end: string; window_hours: number;
   avg_per_hour: number; avg_per_active: number;
@@ -504,6 +505,38 @@ function periodRange(p: Period, from: string, to: string): [string, string] {
   }
 }
 
+// ActivityCard — the "activity over time" chart with a granularity selector. Its
+// own state, module-scoped so a parent re-render doesn't reset the choice. The
+// timeline is the chronological month series; day/week/month/year are the cyclical
+// distributions (hour-of-day, day-of-week, day-of-month, month-of-year).
+const ACT_GRAN = [
+  { key: 'timeline', tkey: 'stats.granTimeline' },
+  { key: 'day', tkey: 'stats.granDay' },
+  { key: 'week', tkey: 'stats.granWeek' },
+  { key: 'month', tkey: 'stats.granMonth' },
+  { key: 'year', tkey: 'stats.granYear' },
+] as const;
+function ActivityCard({ stats, t, empty }: { stats: Stats; t: (k: string, v?: any) => string; empty: string }) {
+  const [g, setG] = useState<string>('timeline');
+  const series = g === 'day' ? stats.by_hour : g === 'week' ? stats.by_dow : g === 'month' ? stats.by_dom : g === 'year' ? stats.by_month_yr : [];
+  return (
+    <Card title={t('stats.overTime')} sub={t('stats.overTimeSub')} className="lg:col-span-2" accent="var(--chart-1)">
+      <div className="mb-2 flex flex-wrap gap-1">
+        {ACT_GRAN.map((o) => (
+          <button key={o.key} type="button" onClick={() => setG(o.key)}
+            className={cn('px-2 py-0.5 rounded-md text-[11px] border transition-colors',
+              g === o.key ? 'bg-primary text-primary-foreground border-primary' : 'border-border text-muted-foreground hover:bg-muted')}>
+            {t(o.tkey)}
+          </button>
+        ))}
+      </div>
+      {g === 'timeline'
+        ? <AreaTrend data={stats.by_month} empty={empty} />
+        : <VBars data={series} empty={empty} showValues height={160} />}
+    </Card>
+  );
+}
+
 // ── Table view (the WCAG-clean twin) ─────────────────────────────────────────
 
 function BucketTable({ title, data }: { title: string; data: Bucket[] }) {
@@ -569,6 +602,7 @@ export function StatsPanel() {
         by_mode: arr(raw.by_mode), by_band: arr(raw.by_band), by_band_category: arr(raw.by_band_category), by_operator: arr(raw.by_operator),
         by_station: arr(raw.by_station), by_continent: arr(raw.by_continent),
         top_entities: arr(raw.top_entities), by_year: arr(raw.by_year), by_month: arr(raw.by_month),
+        by_hour: arr(raw.by_hour), by_dow: arr(raw.by_dow), by_dom: arr(raw.by_dom), by_month_yr: arr(raw.by_month_yr),
         rate: arr(raw.rate), gaps: arr(raw.gaps),
         rate_ops: arr(raw.rate_ops), rate_by_op: arr(raw.rate_by_op),
       } as Stats);
@@ -786,9 +820,7 @@ export function StatsPanel() {
             <StackedBandBars data={stats.by_band_category} empty={empty} />
           </Card>
 
-          <Card title={t('stats.overTime')} sub={t('stats.overTimeSub')} className="lg:col-span-2" accent="var(--chart-1)">
-            <AreaTrend data={stats.by_month} empty={empty} />
-          </Card>
+          <ActivityCard stats={stats} t={t} empty={empty} />
 
           {/* EVERY operator, never a top-N: on a multi-op contest the point is who
               worked what, and a cap would quietly delete the 9th operator. Scrolls

frontend/src/lib/i18n.tsx

@@ -65,7 +65,7 @@ const en: Dict = {
   'stats.byOperatorSub': '“—” = logged by the station owner (no OPERATOR set)',
   'stats.byStation': 'By station callsign', 'stats.byContinent': 'By continent',
   'stats.topEntities': 'Top DXCC entities', 'stats.byYear': 'By year',
-  'stats.overTime': 'Activity over time', 'stats.overTimeSub': 'QSOs per month',
+  'stats.overTime': 'Activity over time', 'stats.overTimeSub': 'Timeline, or by hour / weekday / day / month', 'stats.granTimeline': 'Timeline', 'stats.granDay': 'Day', 'stats.granWeek': 'Week', 'stats.granMonth': 'Month', 'stats.granYear': 'Year',
   'stats.confirmations': 'Confirmations', 'stats.paperQSL': 'Paper QSL',
   
   'stats.byContinentSub': 'Share of the log',
@@ -379,7 +379,7 @@ const fr: Dict = {
   'stats.byOperatorSub': '« — » = loggé par le titulaire (pas d’OPERATOR renseigné)',
   'stats.byStation': 'Par indicatif de station', 'stats.byContinent': 'Par continent',
   'stats.topEntities': 'Top entités DXCC', 'stats.byYear': 'Par année',
-  'stats.overTime': 'Activité dans le temps', 'stats.overTimeSub': 'QSO par mois',
+  'stats.overTime': 'Activité dans le temps', 'stats.overTimeSub': 'Chronologie, ou par heure / jour sem. / jour / mois', 'stats.granTimeline': 'Chronologie', 'stats.granDay': 'Jour', 'stats.granWeek': 'Semaine', 'stats.granMonth': 'Mois', 'stats.granYear': 'Année',
   'stats.confirmations': 'Confirmations', 'stats.paperQSL': 'QSL papier',
   
   'stats.byContinentSub': 'Part du journal',

frontend/wailsjs/go/models.ts

@@ -3927,6 +3927,10 @@ export namespace qso {
 	    top_entities: Bucket[];
 	    by_year: Bucket[];
 	    by_month: Bucket[];
+	    by_hour: Bucket[];
+	    by_dow: Bucket[];
+	    by_dom: Bucket[];
+	    by_month_yr: Bucket[];
 	    window_start: string;
 	    window_end: string;
 	    window_hours: number;
@@ -3967,6 +3971,10 @@ export namespace qso {
 	        this.top_entities = this.convertValues(source["top_entities"], Bucket);
 	        this.by_year = this.convertValues(source["by_year"], Bucket);
 	        this.by_month = this.convertValues(source["by_month"], Bucket);
+	        this.by_hour = this.convertValues(source["by_hour"], Bucket);
+	        this.by_dow = this.convertValues(source["by_dow"], Bucket);
+	        this.by_dom = this.convertValues(source["by_dom"], Bucket);
+	        this.by_month_yr = this.convertValues(source["by_month_yr"], Bucket);
 	        this.window_start = source["window_start"];
 	        this.window_end = source["window_end"];
 	        this.window_hours = source["window_hours"];